1. Field of the Invention
The present invention relates to a fixing heater and to an image fixing apparatus incorporating the same.
2. Description of the Related Art
A fixing heater is incorporated in the heating unit of a printer, for example, to provide heat for fixing a toner image formed on a recording medium such as a paper sheet. Toner used for monochromatic print includes resin and a black-coloring material (carbon black for example). Thus, to fix the toner image onto the recording medium, the fixing heater needs to quickly melt the resin material in the toner, and to continue the heating for over a certain period of time so that the melted resin is fixed to the recording medium.
JP-A-11-338293 discloses a fixing heater with two heating resistor strips formed in parallel on a substrate. The heating resistor strips, having the same width, are both narrow. Accordingly, the heating resistor strips have high resistance, which is advantageous in generating sufficient heat per unit area. However, the narrow width makes it difficult to ensure a large heating area required for having an appropriately long heating time.
JP-A-7-192856 discloses a heater with three parallel electrode strips formed on a substrate, the electrode strips being bridged by a number of heating resistors extending transversely to the electrode strips. With the heating resistors arranged in parallel to each other, the disconnection of any one of the heating resistors does not prevent the other heating resistors from continuing to provide heat. Further, since each heating resistor extends transversely to the electrode strips, the heater can have a large heating region as a whole.
However, the heater disclosed in JP-A-7-192856 (used for anti-clogging protection of the print head of an inkjet printer) fails to heat the areas between the adjacent heating resistors due to the parallel arrangement of the heating resistors with respect to the electrode strips. Therefore, when the heater is used as a fixing heater, the recording medium (printing paper) has a lot of portions unheated in the width direction, whereby the image fixing is not performed properly. Further, since a great amount of electric current passes through the parallel heating resistors, the overall power consumption of the heater tends to be quite large.